In videorecorders of this type the colored video signal is normally divided into a brightness signal (Y signal) and a chrominance signal. Before it is recorded on a video tape, the brightness signal is limited to a given frequency band, transformed in a frequency modulation process and modulated on a carrier. For the purposes of being recorded on the video tape, the chrominance signal is transposed into a frequency range that lies below the range of the frequency-modulated brightness signal. Each of the video fields to be transmitted is recorded on an oblique track of the video tape. The oblique tracks are densely packed without any interstices between them and the lines of the video fields of adjacent oblique tracks are so arranged on the oblique track that the lines of adjacent oblique tracks are completely parallel to each other. With a view to avoiding crosstalk between adjacent magnetic tracks, the two magnetic heads that are provided for both recording and reproduction purposes have their head slit set at a different inclination with respect to the oblique track, which ensures that the video head will be effective only for the oblique track that is to be scanned and makes it relatively ineffective for the oblique tracks adjacent to the track that is to be scanned. However, this ineffectiveness is lost in the range of frequencies with which the chrominance signal is recorded on the video tape.
With a view to counteracting this crosstalk set up during the scanning of the chrominance signals from the video tape, it is customary to arrange a comb filter in the chrominance signal channel on the playback side, the comb filter--in the simplest case--consisting of a two-line retarder circuit and a signal adder circuit. In combination with a repeating sequence of the phase position of successive lines of the recorded video fields, the crosstalk signals of the added useful signals are substantially eliminated in this comb filter.
For illustration purposes, FIG. 1 shows the block diagram of the principal components of the transmission circuit for a color video picture on the playback side of known commercial videorecorders. A video head 3, which is selected line-by-line by an appropriate head switcher 2, scans the recording of an oblique track of the video tape and transmits it via a head amplifier 4 to a brightness signal channel 5 and a chrominance signal channel 6 of the transmission circuit 7 on the playback side of a videorecorder. The circuit arrangements for processing the brightness signal are combined into a single block 8 in this figure. This block supplies the BAS part of the color picture signal. For the purpose of separating the chrominance signals, the chrominance signal channel 6 is provided with a low-pass filter 9, which is followed on the downstream side by a mixer circuit 10 with the connected carrier generator 11 for reconverting the chrominance signal into the standard frequency position and by a band-pass filter 12 for transmitting this standard chrominance frequency to a comb filter circuit 13 in the chrominance signal channel 6. The comb filter 13 usually contains a retarder circuit 14 that causes a delay of two lines and an adder circuit 15 for the phase-dependent addition of the retarded and the unretarded chrominance signal. Even though the signal may arrive at the input 16 of the comb filter 13 with a crosstalk disturbance, at the signal output 17 of the filter it will be substantially free of crosstalk disturbances.
However, a comb filter of this type will work perfectly only for as long as the tonality (chrominance) of the two chrominance signals brought together by the adder circuit is roughly the same. Given a vertical transition of two color areas of complementary tonalities or of a color area in a white surface at a horizontal transition edge, the two-line retardation of the comb filter will cause a disturbing off-color strip to appear at the horizontal transition edge between the two color areas. Disturbances of this kind will now be discussed by reference to FIGS. 2 and 3. FIG. 2, albeit schematically, represents a color video picture 18 on the playback side, showing the two picture excerpts 19 and 20, both of which comprise horizontal color edges. The reference numbers Z6 to Z10, as also Z4 to Z18 in FIG. 3, are arbitrary designations of adjacent video lines of a video field. The picture excerpt 19 comprises the adjacent lines Z6 to Z10 of a horizontal color strip 21 on a white background 22. A schematic representation of this color strip 21 is shown in picture excerpt 19a, the said excerpt 19a corresponding to the picture excerpt 19 of FIG. 2. The lines Z4 to Z5 and Z11 to Z18 in picture excerpt 19a represent the white background 22 on which the color strip 21 is situated. As a consequence of the mode of action of the comb filter, the reproduction of the recorded color strip becomes distorted in such a manner as to reduce the extension of the color strip 21 in the vertical direction, while the horizontal edges of the color strip become lined with edge strips that extend over two lines and have the tonality of a mixture made up of equal parts of the tonality of the color strip 21 and the tonality of the white background. This is schematically illustrated by picture excerpt 19b of FIG. 3. When a copy is made of the recording on this video tape, the playback of the copied recording will be distorted even further by the comb filter, as is schematically illustrated by picture excerpt 19c. When this second recording is played back, the extension of the color strip in the vertical direction is even more markedly reduced. Furthermore, the horizontal color strip 21 is now flanked on both sides by two two-line off-colored edge strips, the tonalities being such that the intensity of the chrominance of the color strip 21 diminishes from the inside to the outer edges. The playing back of a copy of this recording--at least in the example here considered--will not even achieve the original chrominance intensity of the color strip 21, the tonality intensity diminishing in almost sinusoidal fashion until it eventually merges with the white background, all as shown schematically in picture excerpt 19d of FIG. 3.
Furthermore, each recording causes the centre of the color strip 21 to become displaced downwards by one video line of the field, as can readily be seen from the successive picture excerpts 19a to 19d of FIG. 3. This means that, even after a small number of repetitions of the copying process, small color areas having a tonality that stands out sharply against the surroundings will become so markedly distorted and flattened in color as to constitute a disturbance that the viewer cannot be expected to accept. For this reason it is already barely possible to make a recording of a copied recording of a video tape.
With a view to eliminating the color distortions caused by the comb filter at horizontal color edges, the East German patent specification DDR 206 521 proposes to use a phase comparator circuit arranged in parallel with the two-line retarder circuit to detect horizontal color edges and, whenever such an edge is detected, to separate the second input of the adder circuit from the output of the retarder circuit. This eliminates the distorting effect of the comb filter. According to the aforesaid document, however, this method has the drawback that the phase comparison is not sufficient, because--even in the case of completely identical signals--the phase difference of the signals to be compared at any given and constant retardation time will be frequency-dependent. According to the investigations reported in the said patent specification, the phase difference between the input and the output of the retarder circuit depends on the ratio between the transit (or propagation) time of the retarder circuit and the oscillation period of the signal frequencies of the chrominance signal and cannot be used as a criterion for switching off the retarded channel of the comb filter, not even when there is no difference in the frequency of the input signals made available in temporal sequence. In the aforesaid patent specification it is therefore proposed to equip the comb filter with an adder circuit and an additional subtracter circuit and to apply the output signals of the adder and subtracter circuit to a further adder circuit in such a manner that the comb filter--given chrominance signals added in equiphase or subtracted in antiphase--will transmit the chrominance signal with full comb filter effect via the second adder circuit and that during the course of this transmission only the transmission path associated therewith should actually be available to the second adder circuit. This switching operation can be performed either by means of a threshold switch included in the transmission path or by means of a servoswitch controlled by the signal having the greater amplitude at the output of the subtracter or adder circuit.
The known comb filter circuit just described is therefore designed in such a way that the comb filter effect is obtained over a wide phase range, this range being at least twice as great as that of a simple comb filter. This does indeed eliminate the crosstalk disturbance between adjacent video lines of a video field even in the event of largish phase differences. But the color disturbances at the horizontal edges of a tonality or color intensity transition in the vertical direction due to the mode of action of a comb filter become far more disturbing. These line-parallel color disturbances, which are schematically illustrated in FIG. 3, are not in any way diminished when a known comb filter of the type last described is employed. Rather, the use of this comb filter leads to an enhanced appearance of these line-parallel off-color edge strips, so that in this case the viewer of the color video picture will be obliged to view a considerably disturbed picture. This is all the more unsatisfactory in view of the fact that it is nowadays possible for the color video pictures recorded on a video tape to be played back with a far better reproduction quality of the brightness signal than was previously possible.